For a better understanding of the invention, it is exemplified with respect to a non-limiting, ghost cancellation application. The invention is, by no means, bound by this specific application.
The motivation for developing a system of the invention arises, inter alia, from the development of ghost cancellation technology for TV broadcast reception. Reception of terrestrial TV broadcasts often suffers from many types of interference. These interferences can appear on the TV screen in various forms such as noise ("snow"), ghosts and others. One of the most annoying types of interference is the presence of "ghosts" in a TV image. A "ghost" is actually a lower intensity duplicate of the main image, shifted horizontally relative to the main image. These "ghosts" are created by reflections of the broadcast TV signal from large objects such as mountains and buildings. The reflections of the main signal appear at the TV antenna as delayed or preceded and attenuated duplicates of the main signal. When the TV antenna receives the reflections together with the main broadcast signal they are displayed on the TV screen as "ghosts".
The interference caused by ghosts not only annoys the viewer but also degrades the correct operation of the TV circuits. To facilitate the operation of the TV, synchronization signals are transmitted together with the video signal. Reflections that cause ghosts not only distort the image, but also distort the synchronization signals, thus disrupting the normal operation of the TV.
Until recently, the interference of strong ghosts could not be avoided or corrected. The interference due to ghosts adversely affects the quality of the displayed image on the screen because of the multiple images, or because of the failure of the TV circuits to detect correctly the synchronization signals. Digital ghost cancellation systems developed in recent years can resolve the visible effects of the ghosts and restore the image to its original form. Although these ghost cancellation systems eliminate the ghosts and clear the image, their correct operation depends upon the detection of the synchronization signals, e.g. for a given ghost cancellation application an accurate detection of the horizontal synchronization signal of a predetermined line number is necessary. Failure to detect the latter adversely affects the performance of the ghost cancellation application.
It is thus vital for any ghost cancellation system to detect in a high level of certainty the synchronization signals in the received signal despite the distortions.
There follows a brief description of a TV signal pulses (referred to also as "video composite signal") with emphasis on the horizontal (H.sub.-- sync) and virtual (V.sub.-- sync) synchronization signal constituents, and a brief description as to how the hitherto known H.sub.-- sync and V.sub.-- sync separator operates.
Thus, as is well known, TV signal is normally composed of a series of TV lines concatenated into a continuous signal. The TV separates the continuous signal into separate lines and displays the lines one after another from the top to the bottom of the screen. N lines constitute an image field. At the beginning of every new field, the next line is displayed at the top of the TV screen thereby starting a new image.
A new line is determined according to the horizontal sync, which is a unique signal pulses that separates successive lines. The TV determines the beginning of a new field according to the vertical synchronization signal, V.sub.-- sync, which is a unique signal pulses that separates consecutive fields. The composite video signal is composed of the data constituent together with the H.sub.-- sync, V.sub.-- sync as well as color synchronization constituents. The TV circuit should correctly detect the V.sub.-- sync in the composite video in order to reconstruct the TV image field, and to route it to the screen.
For a better understanding of an H.sub.-- sync sync detection sequence, attention is directed to FIG. 1 which illustrates a composite video signal portion (100) having visible video data constituent (102) that surpasses a blank level (104). As shown, the video signal has an associating color burst constituent (106) for synchronizing the color carrier demodulation.
As shown, the video signal portion (100) includes data of two video lines (108) and (108'), respectively to be displayed on the TV monitor.
Each video line is identified by an H.sub.-- sync signal (110) having a sync level amplitude (112) that drops below the blank levels (104).
As is clearly shown in FIG. 1, the H.sub.-- sync waveform (110) always drops below the blank level while the video signal level (102) always surpasses the blank level. Any time the TV detects a transition of the amplitude below a certain threshold, an H.sub.-- sync is detected. The thresh-old (114) is usually half way between the blank level (104) and the sync level (112) amplitudes. Normally, in order to minimize interference by noise, the composite video signal is first passed through a low pass filter that attenuates the noise and thereafter an edge detection circuit detects the negative transition of the filtered composite video from above, to below the threshold.
The conventional, hitherto known techniques for detecting V.sub.-- sync signal in a composite video signal will now be described with reference to FIG. 2.
Thus, vertical synchronization (V.sub.-- sync) pulses (120) is composed of a first equalization pulses (121) followed by a serration pulses (122) which in turn is followed by a second equalization pulses (124). The first equalization pulses (121) consists of 6 (NTSC) or 5 (PAL/SECAM) waveform half a TV line length each (e.g. waveform (126). The serration pulses (122) consists of 6 (NTSC) or 5 (PAL/SECAM) waveforms half a TV line length each, e.g. (128). The second equalization pulses (124) consists of 5 or 6 for odd or even, respectively (NTSC) or 4 or 5 for odd or even, respectively (PAL/SECAM) waveforms half a TV line long each, e.g. (134). As is well known, the pulses of the first and second equalization series is of a flat signal at the blank level, in which one negative pulse is inserted for each half line. The waveform of the equalization pulses is formed by dropping the signal amplitude from the blank level to the sync level (e.g. from (127) to (129) in pulse (126). The waveform of the serration pulses is a flat signal at the sync amplitude level in which one positive pulse is inserted for each half line. The waveform of the serration pulses is formed by raising the signal from the sync level to the blank level (e.g. from (131) to (133) in waveform (128)). The V.sub.-- sync signal is defined as the negative transition of the composite video signal from the blank level (135) to the sync level (128) at the transition between the last waveform of the first equalization pulses to the first serration waveform in the serration series.
According to hitherto known techniques, e.g. in conventional analog TV circuitry, the V.sub.-- sync signal is usually detected by an analog or digital integrator that operates on the composite video signal. During the video portion (130) and the first equalization pulses (121) of the composite video signal surpasses most of the time a so-called vertical threshold level (136) (determined usually to be halfway between the blank and the sync levels) and accordingly, it acquires a positive value. During the serration pulses the integrated signal resides, most of the time below the specified threshold (136) and accordingly, the integrator acquires a negative value. The resulting positive and negative integrator values are manifested by the integrated signal having a positive value (132) and a negative value (135). The falling of the integrated signal level below the threshold (136) triggers a V.sub.-- sync detection event. According to the prior art the occurrence and the timing of the V.sub.-- sync signal are indicated simultaneously. The time constant of the integrator signal is adjusted to be long enough so as not to allow the integrator value to change much during the first equalization pulses, or the serration pulses thereby attenuating noise interferences. The relatively long integrator time constant, whilst attenuating the noise constituent has an intrinsic shortcoming in that the indication of the V.sub.-- sync signal is delayed as compared to the actual occurrence of the V.sub.-- sync signal. Thus, where the V.sub.-- sync signal occurs at the transition from the blank level (135) to the sync level (131), the actual V.sub.-- sync detection event occurs only afterwards when the integrated signal (135) crossed the threshold level (136) at point (138). Whereas precise detection of the V.sub.-- sync signal timing is not obligatory for the control of the TV display, failing to detect the delayed V.sub.-- sync event within a specified tolerance could result in distorted image on the TV monitor. As will be evident from the description below, the underlying premise that the respective blank level and sync level of the equalization and serration pulses are distinguishable one with respect to the other blurs under noisy conditions, (e.g. appearance of ghost) thereby rendering the specified integration technique error prone.
Thus, ghosts can alter the shape of the composite video signal in the V.sub.-- sync pulses such that normal detection of the V.sub.-- sync by the commonly used technique does not guarantee reliable detection. FIGS. 3(a)-3(b) present an example of the effects of a negative ghost on the equalization and serration pulses. Thus, FIG. 3(a) depicts equalization and serration pulses ((140) and (142), respectively) in a ghost free composite video signal. Conversely, and as is shown in FIG. 3(b), in a ghostly environment, the ghost duplicates the pulses and reduces the amplitude difference between the blank level and the sync level (144,146), vis-a-vis, the respective signals in the counterpart equalization and serration pulses (140,142). The blurring of the blank and sync level as shown in FIG. 3(b), results in slowing down the rate of attenuation of the integrated signal (as compared to the desired illustrated level of attenuation shown in FIG. 2) and consequently the V.sub.-- sync signal detection is further delayed. The extent of the delay may well exceed the duration of half a line length, thus causing the V.sub.-- sync signal detection to be delayed and to fall within the next TV line. The latter error cannot be tolerated in any system that requires a correct count of TV lines from the actual position of the vertical sync. In extreme cases, the distinction between the equalization and serration pulses is blurred to an extent that the V.sub.-- sync signal is simply not detected which result in obvious undesired consequences.
FIGS. 4(a-b) demonstrate the effect of an exemplary strong negative ghost on the H.sub.-- sync. The received signal which is the sum of the main video composite signal and a shifted negative ghost, is completely altered. Hence, the genuine H.sub.-- sync (150 in FIG. 4(a)) is altered ((152') in FIG. 4(b) to the extent that it is no longer below the blank level making it very difficult or even impossible for detection by hitherto known threshold detection techniques. Other situations are also possible where the ghost of the H.sub.-- sync is strong enough to cause an extra false detection of the H.sub.-- sync.